In the Data File Editor (DFED) inside the Net Express IDE you might want to edit a SEQUENTIAL data file in FORMATTED mode.

To view a file in FORMATTED mode you need to go through the process of generating the .STR file to record the record layout to be used when editing the file. This is explained in the Net Express manuals.
 
Once this has been done you can then either double-click on the file name or right-click and select 'Edit' in order to view the file.

However, you may have completed this process and then attempted to edit the file, as shown in the manuals. But instead of immediately seeing the file in FORMATTED mode it brings up the Data File Editor, which asks for the File Organization and the minimum and maximum record lengths.

So even though the .STR file has been generated and stored in the same directory as the data file, why doesn't it automatically display the file in FORMATTED mode?

For the data file to be displayed in FORMATTED mode it also requires that a .PRO (i.e. profile) file is also generated.

As SEQUENTIAL files don't have file header records, it requires a .PRO file in which to store the base file information.

So in the situation described above, when it brings up the Data File Editor, select the File Organization and at least the Maximum record length in the file.
If there are multiple record types in the one file (i.e. multiple 01 levels) then provide the length of the longest of these (i.e. most characters).

Once entered select 'OK' and it should prompt you to save it as filename.PRO.

Once you save it and then go back and edit the file it should display each record using the file layout specified in the .STR file.

In simple terms, the .PRO file must exist, along with the .STR file, in order for FORMATTED editing of a data file to display the data records correctly.